Referring to FIG. 4, when a thermocouple 20 is used for temperature-measurement, one end of a first metal 21 and one end of a second metal 22 are joined to form a temperature-sensing junction 23. The temperature-sensing junction 23 is disposed, for instance, at a measuring point of a process 25. The opposite ends of the first and second metals 21 and 22 are connected to, for instance, a control circuit 26 through terminals 4a, 4b (sometimes, jointly referred to as terminals 4, hereinafter) of a measuring instrument 10. In this case, the opposite ends of the metals 21, 22 form a reference junction 24. If the terminals 4 are kept at the same temperature and joined to the opposite ends of the two metals 21 and 22 through the same electric conductors, then it has been theoretically proved that such terminals 4 are equivalent to a reference junction 24 wherein the opposite ends of the metals 21 and 22 are directly joined, as far as the temperature measurement is concerned. In practice, the relationship between the temperature difference and the thermoelectromotive force (temf) across the measuring junction 23 and the reference junction 24 is given for each of different type thermocouples by a Standard under the condition that the reference junction 24 is kept at 0.degree. C.; for instance, Japanese Industrial Standard (JIS) 1602 gives a temf of 41.276 mV for temperature of 1,000.degree. C. at the measuring junction 23 of a K thermocouple.
In actual measurement, the temperature of the reference junction 24 is that of the terminals 4, and the temf deviates from the value of Standard by such an amount which is determined as a temf between the temperature of the terminals 4 and 0.degree. C. reference junction. For instance, in the case of the above K thermocouple, if both of the terminals 4 are at 20.degree. C., the temf of the terminals 4 for the measuring junction 23 at 1,000.degree. C. will be 40.478 (=41.276-0.798) mV, but not 41.276 mV, and accurate measurement cannot be effected. If, however, the temperature of the terminals 4 is measured and found to be 20.degree. C., a temf deviation of 0.798 mV for the temperature difference of 0.degree. C. to 20.degree. C. can be determined from JIS 1602, and the measured temf can be compensated or corrected for 0.degree. C., i.e., to 41.276 (=40.478+0.798) mV, and the measuring junction temperature of 1,000.degree. C. can be correctly measured.
The above correction for the temperature of the terminals 4 connected to the reference junction of a thermocouple is usually referred to as the "reference junction compensation".
The temperature of the terminals 4 connected to the reference junction 24 varies depending on the temperature of a room where the measuring instrument 10 carrying the terminals 4 is located. To cope with changes in the room temperature, it has been tried to mount a temperature-sensing element 13 on a terminals-holding portion 16 of the measuring instrument 10, as shown in FIG. 2, and temperatures of a number of terminals 4 are represented by a measured value of the temperature sensor 13 mounted on the terminal-holding portion 16. In response to change in such measured value from the temperature sensor 13, the reference junction compensation has been carried out in the above-mentioned manner for the reference junction 24 of the thermocouple 20. A conventional arrangement of FIG. 3 uses a collective terminal rack 14 which is thermally separated from the body of the measuring instrument 10, so that heat generated in the instrument 10 should not affect the temperature of the terminal 4. Further, a heat-dissipating plate 15 with a temperature sensor 13 is mounted on the rack 14. After dissipating heat, the temperature distribution on the heat-dissipating plate 15 becomes substantially uniform, and the temperatures of a number of terminals 4 are represented by the measured value of the one temperature sensor 13.
On the other hand, with the trend of miniaturization of instruments and high-density loading of elements thereon, the number of printed-circuit boards per instrument has increased, and power consumption by elements on one printed-circuit board 1 has also increased. Further the density of elements on individual printed-circuit boards has increased, too. As a result, temperature gradient is caused inside the instrument, and if a large number of terminals 4 are mounted directly on a housing 12 as shown in FIG. 2, the temperature of terminals 4 at higher parts of vertical rows may be different from that of those at lower parts of the vertical rows.
In such cases, it is difficult to make accurate compensation for actual temperature variation at the reference junction 24 of a thermocouple 20 by means of a single temperature sensor 13 of FIG. 2 or a combination of such temperature sensor 13 and a heat-dissipating plate 15 of FIG. 3. Consequently, there has been a problem of increased error in temperature measurement by thermocouple with the advance of miniaturization of instrument and intensification of element loading thereon.
In the case of a separate-type collective terminal rack 14 of FIG. 3, the overall dimension of instrument tends to expand, resulting in a cost increase, in addition to the above-mentioned accuracy limitation in reference junction compensation due to the use of a single temperature sensor in common for many terminals.